This disclosure relates generally to a blade tip clearance probe holder and a method for measuring blade tip clearance. More specifically, this disclosure relates to a blade tip clearance probe holder, which holds a capacitance probe that measures the gap between a blade tip and a blade housing.
Gas turbine engines, steam turbines, aircraft engines, jet engines and other axial flow turbomachinery are typically designed to minimize the radial gaps between the blade tips and the blade housings or cases. Gaps between the blade tips and the cases can reduce efficiency by allowing gas or air to leak into the downstream stages of engine operation. The gaps between the blade tips and the cases are a function of engine speed and temperature, and the gaps changes during engine operation. High operating rotational speeds can cause radial elastic growth in rotating hardware (i.e. blades), resulting in radial blade tip growth. Additionally, high temperatures cause thermal expansion in the case and in the rotating hardware. Currently several inspection methods for determining the gap between the blade tips and the fan cases at operating speed are being used.
One method for determining the gap between the blade tips and the case utilizes a thin metal rod inserted and fastened into an axially drilled bolt, the resulting assembly being inserted into a mount plate attached to the fan case. The end of the rod is located where the blade tips should be. The method requires that the engine is operated for a specified time period after which the amount of wear on the rod is measured to determine the change in the gap between the blade tips and the case. The method is insufficient in that the thin metal rods often bend or break which renders measurement thereof moot. In addition, metal liberated from the thin metal rod, either as pieces or as powder can cause damage to the engine. Further, making these thin metal rods can be both difficult and time consuming because each rod must be custom made using a measurement of distance from the fan case to the blade tip. Further, such a method suffers from errors such as measurement, data recording, and machining. It is often the case that the thin metal rods are made either too short or too long. Short rods do not rub the blade tip, while long rods bend or break.
Another method of determining the gap between the blade tips and the case utilizes an electrical capacitance clearance meter, as described in U.S. Pat. No. 5,166,626. The electrical capacitance clearance meter includes a probe that has a capacitor electrode that is electrically connected to an electrical inductor within the probe that is inserted into the fan case to position its electrode at the inner surface of the casing. Electrical power is supplied through a voltage controlled variable frequency oscillator having a tuned circuit of which the probe is a component. The oscillator signal is modulated by a change in electrical capacitance between the probe electrode and a passing blade tip surface while an automatic feedback correction circuit corrects oscillator signal drift. A change in distance between the blade tip and the probe electrode is a change in capacitance therebetween which frequency modulates the oscillator signal. The modulated oscillator signal is then processed through a phase detector and related circuitry to provide an electrical signal that is proportional to the clearance gap.
Newly designed cases require that the clearance probes pass through two features or parts that move independently of each other in order to effectively measure the gap between blade tips and fan case. Current probe holders, such as those described in U.S. Pat. No. 5,166,626, are suitable for mounting on only a single feature and are not capable of being mounted through two or more independently moving features. For the foregoing reasons, there is a need for a new blade tip clearance probe holder and a method for measuring blade tip clearance.